Flash tanks are generally used to flash a high pressure fluid liquor stream including steam and condensate. A flash tank typically has a high pressure inlet port, an interior chamber, an upper steam or gas discharge port and a lower condensate or liquid discharge port. Flash tanks safely and efficiently reduce pressure in a pressurized fluid stream, allow recovery of heat energy from the vapors produced by the stream, and collect chemicals from the condensate of the stream.
Flash tanks may be used to recover chemicals from chemical pulping systems, such as Kraft cooking systems. Flash tanks are also used in other types of cooking systems for chemical and mechanical-chemical pulping systems. To pulp wood chips or other comminuted cellulosic fibrous organic material (collectively referred to herein as “cellulosic material”), the cellulosic material is mixed with liquors, e.g., water and cooking chemicals, and pumped in a pressurized treatment vessel. Sodium hydroxide, sodium sulfite, and other alkali chemicals can be used to “cook” the cellulosic material such as in a Kraft cooking process. These chemicals tend to degrade and solubilize lignin as well as hemicellulose and cellulose compounds in the cellulosic material. The Kraft cooking process is typically performed at temperatures in a range of 100 degrees Celsius (100° C.) to 170° C. and at pressures at, greater than, or substantially greater than atmospheric pressure, such as above 5 bar gauge to 15 bar gauge. In other conventional cooking processes, the cellulosic material may be treated with water or an acid to initiate acid hydrolysis with the focus of solubilizing primarily hemicellulose compounds.
The cooking (reactor) vessels may be batch or continuous flow vessels. The cooking vessels are generally vertically oriented and may be sufficiently large to process 1,000 tons or more of cellulosic material per day. In continuous flow vessels, the material continuously enters and leaves the vessel, and remains in the vessel for multiple minutes or as much as several hours. In addition to the cooking vessel, a conventional pulping system may include other reactor vessels (such as vessels operating at or near atmospheric pressure or pressurized above atmospheric pressure) such as for impregnating the cellulosic material with liquors prior to the cooking vessel. In view of the large amount of cellulosic material in the impregnation and cooking vessels, a large volume of black liquor is typically extracted from these vessels.
The black liquor generally includes the cooking chemicals and organic chemicals or compounds, e.g., hydrolysate, residual alkali, lignin, hemicellulose, and other dissolved organic substances, dissolved from the cellulosic feed materials. The black liquor is typically flashed in a flash tank to generate steam and condensate. The cooking chemicals and organic compounds are generally included with the liquid fraction after flashing. The steam formed from flashing is generally free of the cooking chemicals and organic compounds. The liquid fraction may be processed, for example, to recover and recausticize the cooking chemicals. The steam may be used as heat energy in the pulping system.
In conventional flash tanks, the black liquor usually enters through an inlet pipe coupled to an inlet port on a sidewall of the tank. Other conventional flash tanks may position the inlet port on the top of the vessel. The inlet port is typically a circular or oval-shaped opening in the sidewall of the flash tank. Black liquor typically flows from the inlet pipe into the flash tank. The transition from the inlet pipe to the flash tank is abrupt, which causes disruption and turbulence in the liquor flowing into the flash tank.